In a III-group nitride semiconductor optical device, a technique in which an active layer is made into a quantum well structure by alternately laminating a barrier layer and a quantum well layer has been in widespread use. Employing the quantum well structure enables achieving high power of the device.
In a laser of quantum well structure, however carrier separation occurs due to piezoelectric effect to sometimes deteriorate luminous efficiency. FIG. 1 is a view showing an energy band profile of a quantum well comprising InGaN. An electric field is applied to the inside of the quantum well by the piezoelectric effect and a band structure shown in FIG. 1 is formed to spatially separate electrons injected into the quantum well and positive holes. As a result, overlapping of wave function of the electrons and the positive holes decreases and the luminous efficiency is lowered due to reduction of optical transition probability. In addition, when the electrons and the positive holes recombine and emit light, since both are spatially separated, an emission wavelength becomes long.
To overcome the problems, a technique of doping Si (silicon) onto the barrier layer is often adopted in an InGaN quantum well structure (for instance, it is disclosed on page 211 in Appl. Phys. Lett. 72(2) by S. Nakamura et al.). When Si-doping is performed in the barrier layer, electrons are released into a crystal, and the electrons are distributed in the quantum well layer to block off a piezoelectric field. Prior art of performing Si-doping onto the barrier layer will be described below.
FIG. 2A to 2E are examples of the prior art of an active layer having Si-doped quantum well structure in a gallium nitride based semiconductor optical device. In FIG. 2A to 2E, shaded areas express Si-doped n-type regions and the other areas express undoped regions.
FIG. 2A shows a conventional Si-doped quantum well structure disclosed in Japanese Laid-Open Patent Publication No. 2000-133883, wherein Si is uniformly added as n-type impurity to a whole barrier layer, a quantum well layer is undoped, and the undoped quantum well layer and the barrier layer are combined, thereby enabling to improve of photoelectric conversion efficiency with low Vf and small leakage current and to obtain excellent light emitting output even with low power consumption. In the structure, however, the quantum well layer is formed on the Si-doped barrier layer, thereby sometimes causing an increase of dot defects in the quantum well layer and a rough boundary face of the quantum well layer and the barrier layer. Such deterioration in crystallinity becomes a trigger for reduction in the luminous efficiency.
FIGS. 2B and 2C show a conventional Si-doped quantum well structure disclosed in Japanese Laid-Open Patent Publication No.2000-332364. Each barrier layer is doped with Si, and a distribution (gradient) of Si concentration is formed in the interior of each barrier layer and in a through-thickness direction thereof. The distribution of the Si concentration is different in the cases of FIG. 2B in which an outermost surface is a Ga (gallium) surface (c-plane) and of FIG. 2C in which the outermost surface is an N (nitrogen) surface (-c-plane).
In the case that the outermost surface is the N (nitrogen) surface, when Si-doping is performed into each barrier layer, doping concentration is decreased as a p-type region side is got closer to from an n-type region side. In the case that the outermost surface is the Ga (gallium) surface, the doping concentration is decreased as the periphery of the n-type region side is got closer to from the p-type region side in a direction opposite to the case of N face. The gradient of the Si concentration in the through-thickness direction of a film enables effective reduction of the piezoelectric field. The Ga (gallium) surface (c-plane) and the N (nitrogen) surface (-c-plane) have a difference in their structure shown in FIGS. 3A and 3B, respectively, where a shaded area in FIG. 3A is the Ga surface and a shaded area in FIG. 3B is the N surface. Directions of the piezoelectric field generated in the Ga surface and the N surface are opposite each other. The above-mentioned publication discloses a doping profile such as FIGS. 2B and 2C in consideration of the direction of the piezoelectric field. The result of discussion by inventors of the present invention, however, confirms that the conventional structure is unable to sufficiently eliminate effects of the piezoelectric field.
FIG. 2D shows a conventional Si-doped quantum well structure disclosed in Japanese Laid-Open Patent Publication No.H11-3404559. According to the art described in the publication, a large amount of Si is doped into a barrier layer in a profile such as FIG. 2D, thereby generating an electric field in a direction opposite to the piezoelectric field between upper and lower barrier layer regions (A and B in the drawing) with sandwiching the quantum well layer, and resulting in preventing carrier separation caused by the piezoelectric field. However, it is necessary to dope a large amount of Si into the barrier layer in order to generate the electric field in the opposite direction having an electric field intensity in a level to negate the piezoelectric field. A paragraph (0010) of the publication describes that in the case of GaN, doping of 1E19 cm−3 or more is necessary, and an embodiment shows an example that impurity of 2E19 cm−3 or more is introduced. However, in the case that such a large amount of Si is introduced into the barrier layer, a lowering in a luminous efficiency such as an increase of defect in the active layer and a shortening of light emission life time is sometimes caused.
FIG. 2E shows a conventional Si-doped quantum well structure disclosed in Japanese Laid-Open Patent Publication No.2001-102629. As shown in FIG. 2E, Si is partially doped into each barrier layer, whereby the each barrier layer has a structure that an n-type doped region D is sandwiched by undoped regions C and E. Employing such a structure enables reduction in forward voltage without deterioration in property of the device. However, it has been difficult in the structure that effects of the piezoelectric field are sufficiently eliminated.